Coagulation
Coagulation of blood occurs by either the "intrinsic pathway" or the "extrinsic pathway", whereby certain blood proteins interact in a cascade of proteolytic activations to ultimately convert soluble fibrinogen to insoluble fibrin. These threads of fibrin are cross-linked to form the scaffolding of a clot; without fibrin formation, coagulation cannot occur.
The intrinsic pathway consists of seven steps: (1) the proteolytic activation of Factor XII; (2) activated Factor XII cleaves Factor XI to activate it; (3) activated Factor XI cleaves Factor IX, thereby activating it; (4) activated Factor IX interacts with activated Factor VIII to cleave and activate Factor X; (5) activated Factor X binds to activated Factor V on a membrane surface, which complex proteolytically cleaves prothrombin to form thrombin; (6) thrombin proteolytically cleaves fibrinogen to form fibrin; (7) fibrin monomers assemble into fibrils, which are then cross-linked by Factor XIII.
The extrinsic pathway consists of the following steps: (1) upon rupture of a blood vessel, Factor VII binds to tissue factor, a lipoprotein present in tissues outside the vascular system; (2) Factor VII is activated to Factor VIIa by proteolytic cleavage; and (3) the Factor VIIa-tissue factor complex cleaves and activates Factor X. Thereafter, the extrinsic pathway is identical to the intrinsic pathway, i. e. the two pathways share the last three steps described above.
One of the plasma proteins, coagulation Factor IX ("CFIX") is synthesized in the liver by hepatocytes as a 415 amino-acid polypeptide and then post-translationally modified to a glycoprotein of molecular weight 56,000 Daltons by a carboxylase requiring vitamin K as a cofactor. CFIX is thus one of the group of "vitamin K-dependent" plasma proteins.
Factor VII is another vitamin K-dependent clotting protein that is similar to CFIX in size and structure.
Factor VIII, a non-vitamin K-dependent protein, is a much larger protein, with a molecular wieght of near 300,000 daltons (300 kDa). It is activated by thrombin, which cleaves the molecule in several places to form Factor VIIIa (the activated form). In plasma, Factor VIII binds to von Willebrand Factor (vWF) and circulates as complexes with vWF, which stabilizes the labile Factor VIII molecule.
Disturbing the balance of the cascade involved in the intrinsic pathway results in various coagulation disorders. The absence or reduction of an intrinsic Factor X-activating moiety (a "tenase") at step (4) results in the defective-coagulation condition known as hemophilia. Hemophilia A, the most common, results from a mutation in the gene for Factor VIII; Hemophilia B, also known as Christmas Disease, results from a mutation in the gene for Factor IX. Hemophilia B, like Hemophilia A, is X-linked and accounts for approximately 12% of hemophilia cases. The symptoms are identical to those of Hemophilia A: excessive bleeding upon injury; and spontaneous bleeding, especially into weight-bearing joints, soft tissues, and mucous membranes. Repeated bleeding into joints results in hemarthroses, causing painful crippling arthropathy that often necessitates joint replacement. Hematomas in soft tissues can result in pseudo tumors composed of necrotic coagulated blood; they can obstruct, compress, or rupture into adjacent organs and can lead to infection. Once formed the hematomas are difficult to treat, even with surgery. Recovery of nerves after compression is poor, resulting in palsy. Those bleeding episodes that involve the gastrointestinal tract, central nervous system, or airway/retroperitoneal space can lead to death if not detected. Intracranial bleeding is a major cause of death in hemophiliacs.
Current treatment of these symptoms consists of intravenous replacement therapy with Factor VIII or Factor IX concentrates. Treatment of major bleeding episodes is by bolus injection of concentrate. As described above, however, tissue damage remains even after prompt detection and treatment. Prophylactic treatment is recommended to prevent this pain and debilitation. Upon injection, 50% of Factor IX is immediately bound to vascular endothelial cells and/or diffuses into the extravascular space. The remaining 50% has a half life in circulation of approximately 24 hours. These infusion kinetics result in the need for injections once to twice per week or more to maintain minimal therapeutic levels in the plasma. While this regimen is inconvenient and stressful for the patient, it is also not totally effective. Progressive, cumulative tissue damage continues with each bleeding episode prior to the onset of treatment.